434 THE EVOLUTION OF THE METAZOA 



sqiia?/iaturfi^ however, it can be found considerably farther 

 forwards, on the introvert itself. Their skin which is generally 

 cuticularized and which is closely connected with the skin 

 muscle tube shows a rich differentiation, it is richly equipped 

 with sensory organs, and it contains numerous skin glands; 

 even coelom canals can be found growing through this skin. 



As regards the adult form of the Sipunculida it can be stated 

 that we can notice here some similarities with the Annelida 

 which is by no means a surprising fact, because the Sipunculida 

 had evolved in all probability from some ancestors which they 

 had in common with recent Annelida. They also show some 

 peculiarities that can be observed in the Oligomeria as well as 

 some characteristics that can be found only in the Sipunculida. 



As regards the properties the Sipunculida have in common 

 with the Annelida, mention has most frequently been made of 

 the structure of their central nervous system; this is rightly so 

 in spite of the fact that they have preserved the last traces only 

 of a neuromerism as an irregular arrangement of the pairs 

 of side nerves. This circumstance is important because it serves 

 as good evidence that here we do not have an initial state in 

 the development of the neuromerism but rather a discontinua- 

 tion of the segmentation. The emergence of sensory organs 

 must be evaluated as a progressive and a prospective pheno- 

 menon which is closely connected with the cerebral ganglion 

 (the supraoesopheageal ganglion). 



Less significance should be attributed to the supposed 

 similarities which the Sipunculida share with the Annelida in 

 their ontogenetic morphogeny because here w^e find traits 

 which are much more characteristic of the Sipunculida. It is 

 true that the Sipunculida are Protostomia inasmuch as their 

 oral opening coincides locally with the anterior end of a very 

 elongated blastopore, yet here it is formed anew after the 

 blastopore had first grown together. It has repeatedly been 

 emphasized that in my opinion too great a phyletic signi- 

 ficance has been attributed to the formation of the oral open- 

 ing during the ontogeny. 



